The present invention relates to semiconductor processing methods, and in particular to processing techniques for silicon carbide semiconductors.
Silicon carbide (SiC) is well known as an attractive material for electronic devices operating at high temperatures and high power due to its large energy band gap and high breakdown field. SiC also possesses superior mechanical properties and chemical inertness which are of interest in fabricating microelectromechanical systems (MEMS) as well as nanoelectromechanical systems (NEMS) for applications in harsh environments; environments characterized by locations exposed to high temperature, strong radiation, intense vibration, and corrosive and abrasive media. As a consequence, SiC-based MEMS have found applications in, for example, high-temperature sensors and actuators and micromachined gas turbine engines. Furthermore, due to its high acoustic velocity (defined as the square root of a ratio of Young's modulus to mass density [E/ρ]) and extremely stable surfaces, SiC is recognized as a promising structural material for fabricating ultra-high frequency micromechanical signal processing systems. The highly stable physicochemical properties of SiC also improve the performance of high-frequency resonators as the surface-to-volume ratio increases when the resonator frequency scales into the GHz ranges. In addition, SiC has been found to be a biocompatible material.
One of the challenges in fabricating SiC devices is related to the selective etching of SiC films or SiC bulk materials. Unlike silicon (“Si”), SiC is not etched significantly by most acids and bases at temperatures less than 600° C., which makes wet etching of SiC a difficult task to accomplish. Non-standard techniques such as laser-assisted photoelectrochemical etching of SiC have been developed, but they require special equipment and have poor control over the lateral dimensions. Reactive ion etching (“RIE”) is a known technology for patterning semiconductor devices with precise line-width control. Line-width control is a consideration when the device features are in sub-micron scales. Consequently, the RIE of SiC has been utilizing various fluorinated gases such as CHF3, CF4, SF6, and NF3 in combination with oxygen (“O2”). Using these etch chemistries, the etch rate of SiC is slower than that of Si and silicon dioxide (“SiO2”), due to several factors, including the strong bond between Si and C atoms. Moreover, using these etch chemistries, conventional etch masks for RIE such as hard-baked photoresist, silicon dioxide (“SiO2”), and silicon nitride (“Si3N4”) are etched at higher rates than SiC, which necessitates the employment of metals as etch masks. However, metal masks in RIE are known to cause micromasking phenomena, where metal atoms of the mask material are sputtered by the plasma and deposited in the etch field, producing grass-like structures, which are undesirable. In addition, metal masks in RIE result in contamination of the substrate in subsequent fabrication steps, as well as the microfabrication tools, and hence, are commonly prohibited in integrated circuit (IC) processing.
High-density plasmas have been investigated for SiC RIE to increase the etch rate and improve the etch profile. Such plasmas are generated at low pressures, and have low plasma potential but high ionization efficiencies. The process allows independent controls of ion energy and density. High density plasmas are also able to provide sufficient plasma densities to etch sub-micron features while reducing the surface damage. High-density plasma systems such as electron cyclotron resonance, inductively coupled plasma, including decoupled plasma sources and transformer coupled plasmas have been applied using fluorinated chemistry to achieve high SiC etch rates. However, metal masks are still employed when using these etch plasma chemistries.
There is therefore a need for a high-selectivity etching process using nonmetallic masks for SiC device fabrications.